A well known example of the conventional magnetic focusing CRT has an electrode structure consisting of a cathode K, a primary grid G.sub.1, a secondary grid G.sub.2 and an anode A. In the CRT of this type, video or other control signals are generally input for the primary grid G.sub.1 and the cathode K while the anode A is applied by an anode voltage to be generated by the secondary coil of the flyback transformer. For example, the primary side of the flyback transformer is supplied with a horizontal drive pulse to drive the deflecting yoke, while the flyback pulse is supplied to the primary side of the flyback transformer which resonates from the resonating condenser and the deflecting yoke during the horizontal retrace line period, thus causing the secondary coil to produce the necessary anode voltage.
Furthermore, in the CRT of this type a horizontal retrace line pulse is generally applied to the low voltage section of the high voltage coil in order to keep the high voltage uniform, but when enough pulse is built up to keep the high voltage uniform, the focus voltage extracted from the intermediate tap on the high voltage coil tends to fluctuate. In such cases, the pulse length of the flyback voltage is changed by the flyback transformer, or the power voltage is changed in order to obtain an optimum focal point for the CRT.
However, in the described conventional system of changing the pulse length of the flyback voltage, high voltages are generally used, with difficulty in smooth adjustment and insufficient range of adjustment. According to the system of changing the power voltage, power loss is caused with dispersion of the deflection dimensions in the CRT and changes in brightness which disturbs reproduction of appropriate images.
Furthermore, the flyback pulse to be obtained from the primary side of the playback transformer is influenced by the deflecting yoke, the resonating condenser, the flyback transformer and the other components, providing dispersions in the pulse amplitude and the magnitude value, so that the anode voltage to be produced by the secondary side of the flyback transformer is difficult to come within the allowable values necessary for the CRT.
A purpose of the invention is to provide a focus voltage regulation circuit for cathode ray tubes where when the flyback pulse to be derived from the flyback transformer is supplied as a cathode voltage, a grid voltage and/or an anode voltage for the cathode ray tube, the direct current voltage to be obtained by rectifying a part of the flyback pulse is variably provided to variably and relatively regulate the potential difference between the cathode voltage, the grid voltage and the anode voltage allowing an adjustment of the optimum focal point free of any power loss with a simple circuit arrangement.